The direct reduction of iron ore in vertical shaft, moving bed iron ore reduction reactors has long been known. Representative processes for effecting such gaseous reduction are disclosed, for example, in U.S. Pat. Nos. 3,765,872; 3,779,741; 4,150,972; 4,216,011; and 4,338,123. In such systems reduction of the ore has commonly been effected by a reducing gas largely composed of carbon monoxide and hydrogen obtained by catalytic reformation of a mixture of natural gas and steam. In such systems the ore to be reduced is typically fed to the top of a vertical shaft reactor and flows downwardly through a reduction zone thereof in contact with an upwardly flowing hot reducing gas to convert the iron ore to sponge iron. Effluent gas from the reduction zone of the reactor is cooled to remove water therefrom, and in most cases a major part of the cooled effluent gas is reheated and recycled to the reducing zone. At its lower end the reactor is provided with some means for controlling the discharge of sponge iron from the reactor, e.g. a rotary discharge valve, a vibratory chute, conveyor belt or the like.
Catalytic reformers for producing the carbon monoxide/hydrogen reducing gas mixtures for use in such moving bed reactors are relatively expensive and natural gas used as a feed material for the reformer is not always readily available at an acceptable cost. Hence it has been proposed that the reducing gas be generated in a coal gasifier, for example, a melter-gasifier of the type in which powdered coal and an oxidant gas are fed to a molten metal bath. The coal is partially burned by the oxidant gas, advantageously oxygen, to generate heat that maintains the bath molten and to produce a reducing gas suitable for use in the ore reduction reactor. The ash from the combustion of the coal is removed from the gasifier periodically or continuously as a molten slag. Suitable melter-gasifiers of this type are known in the art.
While in general coal gasifiers provide a relatively inexpensive source of reducing gas, their use leads to certain other operating problems. Thus the gas generated in the gasifier is relatively dusty and the dust tends to clog the interstices between the ore particles in the reactor. If the gas is scrubbed with a scrubbing liquid to remove the dust, its sensible heat is lost and the gas must be reheated before being introduced into the reactor. Also, one of the problems encountered in the operation of a moving bed reduction reactor is the tendency of the ore particles to agglomerate and form large aggregates and such agglomeration impedes the flow of solids through the reactor